The ultrasonic propagation properties of living systems, and preparations of their molecular constituents, are studied with the view toward obtaining a more profound understanding of the fate of the wave process in such media and toward providing information to insure effective and safe clinical use of this modality. Special attention is devoted to absorption properties of tissues and the physical mechanisms responsible for loss of energy. Recent findings suggest that mammalian gonads have relatively low absorption coefficients and that structural proteins may provide for greater ultrasonic attenuation than do the globular proteins. Development of instrumentation is also included in the research program. BIBLIOGRAPHIC REFERENCES: J. C. Bamber, M. J. Fry, C. R. Hill, and F. Dunn, Ultrasonic Attenuation and Backscattering by Mammalian organs as a function of time after Excision, Ultrasound in Med. and Biol 3 (1977), in press. F. Dunn and W. D. O'Brien, Jr., Ultrasonic Absorption and Dispersion, in Ultrasound: Its Application in Medicine and Biology, ed. by F. J. Fry (Elsevier, Amsterdam, 1977), in press.